Conventionally, a method has been known for representing the sound transmitted from a speaker to the ears using head-related transfer functions (HRTF(s)). HRTFs are functions that represent how the sound being generated from the speaker (sound source) sounds to the ears. By applying filtering on the sound source such as a speaker using such HRTFs, it is possible to give a person a feeling that there is a sound source in a location where such sound source does not actually exist. This processing is referred to as “localizing a sound image” at the location. The HRTFs can be determined either by actual measurement or by calculations. The successful application of this technology makes it possible to resolve a problem that some people feel as if the sound source existed inside their heads when using a headphone and to produce the effect of giving a sense of realism to the listener listening to the sound from a small stereo equipped to a mobile phone or the like as if such sound were coming from a large stereo.
FIG. 1A is a diagram showing an example conventional method for determining HRTFs by actual measurement. In general, the measurement of HRTFs is carried out inside an anechoic chamber where there is no reverberation of sound from the wall or the floor, using a test subject or a measuring manikin with the standard dimensions called a dummy head. In FIG. 1A, a measuring speaker is placed about a meter away from the dummy head and transfer functions from the speaker to both ears of the dummy head are measured. Microphones are placed inside the respective ears (auditory tubes) of the dummy head. These microphones receive specific sound impulses emitted from the speaker. In this drawing, “A” denotes a response from the ear further from the speaker (far-ear response) and “S” denotes a response from the ear nearer to the speaker (near-ear response). As described above, by recording responses of the microphones to impulses from the speaker, with the speaker moved at various azimuthal and elevation angles with respect to the dummy head, it is possible to determine HRTFs between sound sources at various locations and the respective ears.
FIG. 1B is a block diagram showing the structure of a conventional sound image control device. As shown in FIG. 1B, such sound image control device modifies the HRTFs measured as shown in FIG. 1A by performing signal processing on the time domain and frequency domain. In other words, processing is performed on an input signal for the near-ear response, far-ear response, and inter-aural time delay included in the HRTFs represented by the diagonally shaded block, so as to output headphone signals. Variations among listeners are supported as follows: for a listener whose ear size is larger than the standard dimensions, resonance frequencies of the respective frequency response characteristics of the near-ear response and the far-ear response are reduced according to the ratio of the difference from the standard dimension; and for a listener whose head dimensions are larger than the standard dimensions, a time delay is increased according to the ratio of the difference from the standard dimension. Such technology is disclosed in Japanese Laid-Open Patent application No. 2001-16697 (page 9).
FIG. 2 is a diagram showing an example conventional technology for calculating HRTFs for plural sound sources using a three-dimensional head model represented on a calculator. In order to calculate HRTFs on a calculator, a three-dimensional shape of a head such as a dummy head is loaded into the calculator, so as to use it as a head model. In this drawing, each intersection of the mesh illustrated on the outer surface of the head model is referred to as a “nodal point”. Each nodal point is identified by three-dimensional coordinates. In the case of determining HRTFs by calculations, the potential at each nodal point on the head model is calculated for each sound source (sound emitting point), and the sound pressures of calculated potentials at the respective nodal points are combined. FIG. 2 illustrates the case of determining HRTFs when sound sources are placed at angles of 0 degrees, 30 degrees, 60 degrees, and 90 degrees, respectively, with respect to the right ear of the head model. In this case, it is possible to calculate HRTFs when the sound sources are placed at the angles of 0 degrees, 30 degrees, 60 degrees, and 90 degrees by calculating the potential at each nodal point when the sound source is placed at the 0 degree angle, the potential at each nodal point when the sound source is placed at the 30 degree angle, the potential at each nodal point when the sound source is placed at the 60 degree angle, and the potential at each nodal point when the sound source is placed at the 90 degree angle.
However, such conventional structure requires the measurement of an enormous number of transfer functions in the case of measuring detailed variations in azimuthal and elevation angles. With regard to this, there are the following problems: (1) it is difficult to stabilize a measurement condition each time the location of the speaker is changed; (2) the size of microphones used for measurement cannot be ignored while the size of ear canals is ignorable; and (3) due to such reasons as that the size of the speaker has an affect on the sound field in the case where HRTFs are measured in the vicinity of the head, highly accurate HRTFs cannot be obtained, and thus in the case where an acoustic transducer located in the vicinity of one meter or less away from the head is used, it is difficult to control sound images correctly. Furthermore, also in the case where HRTFs are determined on a calculator, while it is desired to calculate HRTFs with the sound source being placed in a larger number of different locations, there is a problem in that it requires the calculation of the potential of each of an enormous number of nodal points each time the location of the sound source is changed.
There is also a problem in that, since modification of transfer functions according to head dimensions is made by adjusting an inter-ear delay time in the case where the head is regarded simply as a sphere, variations in the frequency characteristics attributable to an interference between sounds that diffract around the head cannot be reproduced and thus differences in the effect of sound image control among individuals cannot be reduced.
The present invention aims at solving the above problems, and it is an object of the present invention to determine enormous kinds of transfer functions for different azimuthal and elevation angles and different distances in a highly accurate manner under the same condition.
A second object is to provide a sound image control device that is capable of obtaining precise localization of sound images even in the case of using an acoustic transducer located in the vicinity of the head by obtaining a highly accurate transfer function even when an acoustic transducer is located in the vicinity of the head.
A third object is to provide a sound image control device that is capable of supporting individual differences in sound interference that varies depending on head dimensions as well as differences in the internal shape of ear canals and thus capable of reducing individual differences in the effect of sound image control.